Equal chromosome segregation in mitosis requires proper interaction between the chromosomes and the mitotic spindle. Kinetochores are the specialized structures that are assembled on centromeric DNAs and mediate the attachment between the chromosomes and the spindle microtubules. Kinetochores of various organisms differ remarkably in their architectures and yet perform fundamentally similar functions. Much has been learnt of the kinetochores in S. cerevisiae. However, it is unclear to what extent can we apply the knowledge of this simple point kinetochore onto complex regional kinetochores such as those of humans. The long-term goal of this proposal is to dissect the kinetochore structure in S. pombe as a model of the regional kinetochores and to understand how kinetochores co-ordinate with the spindle to ensure the high fidelity of chromosome segregation in this model organism. By comparing and contrasting the kinetochores of S. pombe, S. cerevisiae and those of other organisms, we wish to delineate the general biological principles of the kinetochore functions. This knowledge will facilitate the identification of the molecular lesions in tumor cells which lead to high rate of aneuploidy - erroneous chromosome numbers that are detected in virtually all types of tumor. Three specific aims are proposed here to analyze the kinetochore/spindle interaction in fission yeast: 1. To identify novel kinetochore and spindle components in fission yeast that are required for their proper attachment. 2. To characterize the architectural and functional roles of novel kinetochore proteins. 3. To explore the mechanism of how kinetochores switch from a static affinity to spindle pole to a dynamic affinity to spindle MTs at the entry of mitosis.